X-ray security inspection systems are widely used at airports and other security-sensitive locations to scan baggage and other containers for explosives and other contraband. These X-ray security inspection systems typically have one (or a few) fixed (i.e., stationary) X-ray sources running at low power (e.g., 2 mA) at approximately 160,000 volts (i.e., 160 kV). The bags or containers (typically loaded in trays) are moved past the fixed X-ray source(s) on a conveyor belt.
When the X-rays impinge upon a bag or other container passing by the X-ray source(s) on the conveyor belt, moderate amounts of scattered X-rays are emitted from the bag or other container under scrutiny. Some of these scattered X-rays are reflected in the directions of the ingoing or outgoing segments of the conveyor belt which is moving the bags or containers past the fixed X-ray source(s). To avoid X-ray exposure to humans that may be near the entrance to the X-ray security inspection system (i.e., the “ingoing tunnel” containing the ingoing segment of the conveyor belt) or near the exit of the X-ray security inspection system (i.e., the “outgoing tunnel” containing the outgoing segment of the conveyor belt), a number of lead-containing curtains (e.g., 3 to 6 curtains) have hitherto been placed in each of the tunnels (i.e., in each of the ingoing and outgoing tunnels).
In these older X-ray security inspection systems, a typical throughput rate for the trays (also sometimes called bins) containing the bags or other containers has generally been in the range of 200 (or at most 300) trays per hour. Therefore, at these low rates of passage of trays through the ingoing and outgoing tunnels where the lead-containing curtains were located, a tray could move through that relatively low number of curtains with adequate spacing between the trays, and hence with adequate time for the curtains to be pushed up, and then fall back down, between the trays containing the bags or other containers being scanned.
So, in summary, with the older X-ray security inspection systems which involved a combination of low X-ray power and low tray throughput, a modest number of lead-containing curtains were able to adequately attenuate the moderate amount of scattered X-rays being emitted into the ingoing and outgoing tunnels from the bags or other containers carried by the trays.
However, over the past few years, it has become evident to those responsible for airport security that the effectiveness of these older X-ray security inspection systems is grossly inadequate. In response, a number of companies commenced the design of computed tomography (CT) security inspection systems. Such CT security inspection systems are intended to have a throughput rate of approximately 600 trays per hour (i.e., 600 bags or other containers per hour). These CT security inspection systems, which make many X-ray projections (e.g., 1,000 or more projections per rotation of the CT machine), necessarily use higher power X-ray, typically 5-8 mA at 160 kV, and furthermore use about 24 rows of projections simultaneously. As a result, the power of the scattered X-rays from the CT security inspection systems is almost 100 times greater than the power of the scattered X-rays from the older X-ray security inspection systems, and it has become clear that a practical solution needs to be found to reduce the scattered X-rays emanating from the ingoing and outgoing tunnels of the CT security inspection systems.
Initially, it appeared that this problem could be solved by simply adding more lead-containing curtains at the ingoing and outgoing tunnels of the CT security inspection system. However, this is not the case. To understand the magnitude of the problem, consider that, because the CT security inspection systems produce nearly one hundred times more scattered X-rays than the older X-ray security inspection systems, it is necessary to reduce the level of scattered X-rays emanating from the ingoing and outgoing tunnels by a factor of 30,000:1, rather than the previous requirement of approximately 300:1. Note that a curtain with a lead equivalency of 0.5 mm lead thickness attenuates the scattered X-rays by a factor of about 5.5:1. For four curtains, this factor is raised to the fourth power, which results in an attenuation of about 915:1, which is more than enough attenuation for the older X-ray security inspection systems. That is, it took only four curtains of 0.5 mm lead equivalent to adequately shield the ingoing and outgoing tunnels of the older X-ray security inspection systems, but takes at least six such curtains being “fully down” to produce the attenuation of 30,000:1 required by the CT security inspection systems. Furthermore, at the high throughput speeds of the CT security inspection systems, where a tray is almost always disposed under (and displacing) some of the curtains, it takes more than six installed curtains to provide the at least six “fully down” curtains at any given time. However, if this larger number of lead-containing curtains is installed at the ingoing and outgoing tunnels of the CT security inspection system, the lead-containing curtains must be closer to each other (since the length of the ingoing and outgoing tunnels is generally heavily constrained by the space available for the CT security inspection system), and this causes the double problem of (i) the tray must be pushed harder to lift more curtains, and (ii) the curtains do not come “fully down” until after the entire tray has passed by the curtain by a distance which is somewhat greater than at least an additional 30 cm or so (this distance is a function of the height of the trays and the bags or containers loaded in the trays, etc.). So, simply adding more curtains does not work at all for the higher throughput rate of the CT security inspection systems.
See FIGS. 1-5, which illustrate how the lead-containing curtains do not come “fully down” at the higher throughput rate of the CT security inspection systems.
More particularly, FIGS. 1-5 show an exemplary prior art CT security inspection system 5. CT security inspection system 5 generally comprises a CT machine 10 having a rotating focal spot 15 producing a multi-row X-ray 20. Ingoing and outgoing tunnels 25, 30 provide ingress and egress for a conveyor belt 35 to move trays 40 (containing bags or containers) past rotating focal spot 15 of CT machine 10. Lead-containing curtains 45 are disposed in ingoing and outgoing tunnels 25, 30.
With a throughput rate of 600 trays per hour (i.e., one tray every six seconds), and with conveyor belt 35 moving at 15 cm per second (a typical speed to enable the required image quality from CT machine 10), one tray passes along the conveyor belt every 6 seconds. Where each tray has a length of 60 cm, this means that there is a 30 cm spacing between trays on the X-ray conveyor belt (i.e., a belt speed of 15 cm per second and one tray every six seconds equals 90 cm between trays and, with each tray having a length of 60 cm, this yields 30 cm spacing between trays). However, with only 30 cm spacing between trays running on a belt moving at 15 cm per second, there is insufficient time for a displaced lead-containing curtain to come back down to its “fully down” position between trays. Thus, with a throughput rate of 600 trays per hour, and with a conveyor belt speed of 15 cm per second, the lead-containing curtains of the CT security inspection system cannot adequately shield scattered X-rays passing through the ingoing and outgoing tunnels of the CT security inspection system. This issue is discussed in greater detail below.
Another approach proposed for attenuating X-rays exiting the ingoing and outgoing tunnels of a CT security inspection system was to make the ingoing and outgoing tunnels much longer so that the lead-containing curtains could be spaced much farther apart. In theory, this approach might give the curtains time to come “fully down” between successive trays, but in practice it requires the ingoing and outgoing tunnels to be excessively longer than is generally allowed by the space constraints present at airports and other security-sensitive locations.
Another approach for attenuating X-rays exiting the ingoing and outgoing tunnels of a CT security inspection system is described in U.S. Patent Application Publication No. US2016/0372223 A1. The approach of U.S. Patent Application Publication No. US2016/0372223 A1 uses rotating curtains between successive trays. However, in practice, this approach does not work because the rotating curtains need to be precisely synchronized with tray movement in order to quickly come down between incoming trays. Furthermore, this approach does not work at the higher, desired throughput rate of the newer CT security inspection systems, since it is difficult to get enough curtains fully down between the trays to provide the required level of X-ray attenuation.
Still another proposed approach was to provide multiple curtains on reels going rapidly up and down between successive trays. Aside from the engineering complexity and power requirements needed to make the curtains go up and down fast enough, such an approach is excessively costly, adding significant expense in each ingoing and outgoing tunnel.
Yet another proposed approach was to build a complicated contrivance which would open the ingoing tunnel for a short period of time, push the tray in rapidly for scanning, reverse the process at the outgoing tunnel, and then repeat the process for the next tray. While theoretically possible, the difficulty, reliability and power required by such a complicated system has been found to be impractical.
Yet another approach might be, for a given desired throughput rate, to significantly speed up the conveyor belt speed through the system. This approach would allow the trays to go through the system faster, with more space between the successive trays, so that the lead-containing curtains in the ingoing and outgoing tunnels will have time to come down between successive trays. However, to obtain the same image quality at this higher throughput speed requires a proportionately higher power X-ray source or, alternatively, a significantly larger number of detector rows, or both, and it requires speeding up the rate of rotation of the gantry carrying the X-ray source of the CT scanner. Thus, this approach adds significantly to cost, and increases the power requirements for the system (which is generally not available at the airports and other security-sensitive locations in which the CT security inspection systems must be installed).
Thus there is a need for a new and improved computed tomography (CT) security inspection system with enhanced X-ray shielding.